The development in the radio communication field during the recent years has created an increased need for high-power radio frequency amplifiers. One reason for this is the increased use of modulation schemes using time-dependent envelopes, like QAM (Quadrature Amplitude Modulation), OFDM (orthogonal frequency division modulation), and CDMA (Code Division Multiple Access). Another reason is the development towards multi-carrier radio (MCR).
In cellular networks, terminals can be connected to radio base stations. In a radio base station, there is a need for a linear nigh-power amplifier in the transmitter section to provide each radio channel with sufficient power to reach the outer limits of the cell covered by the base station. Traditionally there has been a trade-off between efficiency and linearity in radio frequency power amplifiers. E.g. C-type amplifiers have a high efficiency but have an insufficient linearity, whereas e.g. A-type amplifiers are very linear but have a low efficiency.
When the same amplifier is used for simultaneous amplification of several information signals modulated on different carrier waves or when linear modulation is used, such as QAM, a high linearity is required. This is because, in this case, it is essential that all phase and amplitude positions of the signal components involved are maintained in the amplification. If many carriers are amplified in a single amplifier, the envelope of the total signal will be time-dependent even if the individual signals are not. If linearity is not achieved, inter-modulation between the signal components might occur or the spectrum of the amplified signal sum might broaden, resulting in an interference with signals on other channels. It has, therefore, been particularly problematic to find solutions for e.g. MCR (Multi-Carrier Radio) base stations maintaining a high efficiency due to the very stringent linearity requirements at the same time as high power is needed. In addition, the relatively large bandwidth makes a solution for this case particularly difficult.
The published International patent application WO 98/11683, inventors L. Hellberg et al., discloses a method for generating a moderately wide-band (i.e. including a MCR signal) high-power RF signal with a high efficiency and linearity. In this method, a sigma-delta modulator is used to generate a digital signal from an information signal followed by digital up-mixing and subsequent switching and band-pass filtering. The sigma-delta modulation transforms the analog (or highly multi-level digital) signal to a signal containing only M (preferably, but not necessarily, equally spaced) levels by a quantization process. A band-pass filter then rejects the so-called quantization noise generated in this process. The switching process provides the amplification. The input of the band-pass filter is connected to M different constant electrical potentials via M switches. At a given time, one and only one switch is closed, and all the others are opened. The digital control signal (the digitally up-mixed sigma-delta coded base-band signal) determines which switch is closed. The sigma-delta amplifier has a switching frequency equal to twice the “carrier” frequency. The switches are connected to DC voltages.
If ideal switches (and an ideal band-pass filter) are used, the amplifier would have a 100% efficiency and linearity. Real switches are, however, not ideal and therefore they will dissipate power and a 100% efficiency is not obtained. Power will be dissipated due to voltage-drops across the closed switches and leakage current through the opened switches.
Furthermore, the switches have finite transition times between the closed and opened states. This results in a problem of switching transients. If, during some period during a switching transient, two switches are simultaneously closed, i.e. are in a low impedance state, an almost short-circuited power supply would result.
If, on the other hand, during a switching transient, all switches are simultaneously open, i.e. are in a high-impedance state, the band-pass filter would create a voltage transient. The band-pass filter must have a high input impedance for out-of-band signals. For this reason, it can be considered as a constant current generator during the short interval of time of a switching transient. This constant current passes the high-impedance switches, creating a high-voltage transient. This dissipates transient power, may create non-linearities, and even degrade the lifetime of the switches. It will also be more difficult to get the required selectivity from the band-pass filter since it will see a time-varying impedance.
One way to reduce the transient problems could be to use a faster switch. However, there is often a trade-off between speed, the conductivity of the closed switches (for a given minimum “off-resistance” in the opened state), and the required control-signal power. Thus, small transient losses may imply e.g., higher ohmic losses in the steady state “closed” state (or higher control-signal power).